here's what he has in brawl that he didn't have in melee
1. smashes - given, you could work in a fsmash to fake people out in melee, and usmash wasn't horrible, but donkey kongs new smashes ****. usmash destroys the light guys, especially if platforms are included, they die in the late 50's or early 60's. fsmash, when spaced properly, is one of the best moves in the game, its reach is longer than most people's rolls, so if they see it coming and roll, you get to charge it and punish, on top of this, it can hit people who stay on the ledge too long, and eats the roll off the edge, the jump off the edge, and most of the attacks off the edge if you time it right. and the dsmash is nearly broken, as mikehaze put it "all you have to do is dsmash and roll" it can do everything, it's basically the fsmash for people closer than you'd like
2. the third best approach in the game - warios air control, bike, superarmor and priority take first by a long shot, and marth's sideb and double fair give him some great options, but dk's super armor and double bair give him one of the best approaches in the game (you could argue metaknights is better than dk, however, metaknight's can be predictable) and make it nearly impossible to camp dk, which was one of his downsides in melee
3. he too can "airdodge 100 times" - dk got combo'd hard in melee, and with brawls physic/mechanics, thats gone, you can just airdodge back to the ground.
4. he's one of the few who can punixh effectively in this game - in melee, anyone could punish a mistake, in brawl, certain characters can do it much easier than others, and dk is one of those select few. if your opponent likes to roll, dsmash and fsmash fix that quick, if the other play wants to spam mach tornado or the turtle, or whatever high priority, quick move his character has, the bpunch will stop that, if he airdodges your uair when you try to keep him in the air, you canfastfalland either bair him, or nair him, depending on his movement, both will knock him off the ledge.
5. new edge guarding - upb still works, and can now be auto canceled on the ledge, also, you can jump out and spike much easier, and bair ***** off the edge, nair works too depending on the character. and if you miss upb kills, bair to upb kills, you just gotta chase them off. edgeguarding/gimping has moved from holding the ledge and smacking them when they get near to jumping waayyy out and hitting them with a good attack or two,which dk can do with his huge horizontal recovery
6. usable downb/sideb - self explanitory
here's what he is presumably missing
1. juggling with uair - utilt under a platform works for most characters, utilt in general works for fastfallers and/or people with laggy start up time on aerials like samus or ness, if they airdodge,you can get off another utilt before they hit the ground
2. bpunch range - it now has super armor instead, and fsmash has more range than the punch did in melee
3. upb edge guarding - its preference really, i can edge guard better, given, my melee dk was not remarkable, but there weren't many good dks in socal so i can't really compare it
4. good match ups against the two best character - now he has good matchups with 3 out of the 4 best character (4 out of 5 if you count wolf up there) with wario being the only one that really gives dk problems, but wario gives everyone problems
5. chaingrabs - most people are missing this, he does have very good throws compared to the rest of the cast in brawl